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Notes on Geologic Time

Geologic time can be beyond human comprehension. Sure, geologists have broken it into units: periods, epochs and stages, but it is still mind-numbingly vast. And at the same time, just a bit facile.

Let’s use the Oligocene Epoch as an example. On the one hand, in terns of the history of the planet, the period from 33.9 million years ago (MYA) to 23.03 MYA, about 11 million years, is a tiny, tiny fraction of the 4.6 billion years the Earth has existed. About a quarter of one percent. Stretch your arms out to full length. Your arm span is the history of the earth, from zero at the tip of the fingers of your left hand, and today at the tip of the fingers of your right hand. The Oliogcene starts at about the outermost knuckle of your right hand. And lasted about a quarter of the way across that last part of your finger. It’s pretty trivial in terms of the Earth’s history.1

But that the same time, it’s 11 fricking million years and it ended 23 million years ago. To a human, who might live 80 or 90 years, it’s an amount of time that is nearly unimaginable. Recorded human history is maybe 5,500 years. One brief geologic epoch lasted 20,000 times as long as all of recorded human history.

So when someone says there was a lot of volcanism in the Pacific Northwest in the Oligocene, what they mean is there were some dramatic volcanic episodes with long, long stretches of time between those volcanic episodes. A long enough time for volcanic rock and ash to weather to soil, for the soil to be buried under still more volcanic ash. Where those buried soils could be fossilized, forming paleosols, literally fossilized soil. For those volcanic rocks and soils to then be eroded by rivers and deposited as sandbars on long-vanished rivers, only to be buried by still more volcanic ash and magma.

The Painted Hills from the Overlook, Painted Hills Unit, John Day Fossil Beds Nat’l Monument, Oregon

The Painted Hills in the John Day Fossil Beds are a reproof and caution to geologists who are too facile about geologic time. It takes a long time for weathering processes and plant life to erode congealed lava to soils. On the Big Island of Hawai’i, Highway 270 winds around the northerly end of the island. The lava looks fresh, like it just congealed. In fact, it’s some of the oldest rock on the Big Island, more than 140,000 years old. Even 140 millennia later, it’s still a long, long ways from being top soil.

In the John Day Formation, the bands of different colors are bands of paleosols, fossilized soils. The color changes are different climate regimes; the reds represent warmer and wetter periods. That’s a whole lot of geologic time stacked up in that hillside.

Maybe it’s a kind of subconscious compression, but the colored bands of the Painted Hills represent hundreds of thousands of years where there was no volcanism, where a subtropical climate slowly turned rock into dirt. The volcanism gets all the press, all the studies and all the attention. But cataclysms are the exception, not the rule. Over most of geologic time, every day is pretty much like the day before or the year before.

Uniformtarianism and Sir Charles Lyell were right, mostly. It’s just as well; the Earth is exciting enough as it is. In the barely comprehensible scale of geologic time, there’s ample time for volcanic cataclysm. The evidence is right there.

As John McPhee pointed out, if you were careless with a nail file on your right hand, three strokes of it would obliterate all of human history. ↩